1. Field of the Invention
The present invention relates to an acoustic meter assembly and in particular to a holder for an acoustic transducer employed in such a meter.
2. Description of the Prior Art
Acoustic meters, such as flow meters, gas composition meters, etc. are well known in the art. Such meters typically employ one or more ultrasound acoustic transducers arranged within a measurement chamber for receiving a gas or other fluid, the flow, composition, or other property of which is to be measured. The one or more ultrasound transducers operate to transmit an ultrasound signal into and subsequently receive a dependent ultrasound signal from fluid within the measurement chamber and the flow or other property is determined from differences between the transmitted and the received ultrasound signals. To accomplish this the ultrasound transducer typically is mounted in a holder having a transducer receiving section either connectable to or more usually integrated with an opening in the measurement chamber. A rubber flange is provided which forms a fluid-tight seal between the transducer and the receiving section and acoustically isolates the transducer from the walls of the chamber itself. In this manner a front face of the transducer is exposed to fluid within the chamber.
In order to make accurate determinations using an acoustic meter of the above-described type it is essential that the acoustic path length within the measurement chamber be known accurately. However, the exposed front face of the transducer is subject to any pressure changes of the fluid within the chamber. During inspiration, for example, a transducer employed in the measurement of inspiration gas may be subject to approximately 100 mbar pressure increase at the front surface. Such an increase exerts a pressure force on the front surface that is likely to cause the transducer to move with the rubber flange by approximately 12-13 μm and to thereby increase the acoustic path length by around 25 μm. This increase will lead to an error in the determination made using the meter which will increase the shorter the path within the measurement chamber that is traversed by the acoustic energy becomes. Unfortunately, a short acoustic path, typically of 4 cm or less, is often desirable since this will reduce the gas volume required as well as making any necessary gas temperature measurements easier.
One known solution is to design a holder for the transducer having a biasing element, such as a spring, in contact with a rear face of the transducer. This spring provides a force on the transducer that is counter the force exerted on it by fluid pressure from within the measurement chamber and so inhibits pressure induced movements of the transducer. Unfortunately, a large bias force often has adverse effects on the operation of the transducer and it becomes extremely difficult to provide a bias force that is sufficiently large to prevent the small (micron) movements of the transducer which effect the accuracy at the small acoustic path lengths.